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JULY 28, 1769 (M=6)
[c6, p156]

The earthquake history of California serendipitously begins with the first overland expedition through the State in 1769. In response to the perceived threat posed by Russian expansion into the northern Pacific and growing British presence in the northwestern Pacific, Spain embarked on the colonization of present-day California through the establishment of a series of Franciscan missions, supported by military garrisons at San Diego and Monterey. In the summer of 1769, Gaspar de Portola led the first expedition from San Diego to establish a land route to Monterey.

On July 28, while camped along the Santa Ana River, about 50 km southeast of Los Angeles, a sharp earthquake was felt that "*** lasted about half as long as an Ave Maria" ( Figure 6.3). From the diaries of three members of the expedition, we know that earthquakes were felt on nearly a daily basis through August 3, as the party traveled northwestward to near San Gabriel and then westward across Los Angeles to the Pacific. The diary of Fray Juan Crespi (Bolton, 1927) mentions no fewer than a dozen aftershocks, some described as violent. After August 4, no further earthquakes were mentioned as the expedition traveled into the San Fernando Valley and exited to the north.

These sketchy reports suggest that the explorers traveled near or through the epicentral area of a moderate earthquake (Richter, 1973; Toppozada and others, 1981). Comparisons between the accounts of the aftershocks and more recent events suggest an event of similar size and location to the 1933 Long Beach, 1971 San Fernando, or 1987 Whittier Narrows earthquake. If significance is placed on the absence of aftershocks while crossing the source region of the 1971 San Fernando earthquake, the evidence would seem to favor a source in the Los Angeles Basin. An event on either the San Andreas or San Jacinto faults, some 50 km to the northeast, could conceivably have been the source of the 1769 earthquake. The description of the duration of strong shaking, however, suggests a magnitude more of 5-6 than of 7-8.

A more distant source would make the long, felt aftershock sequence even more remarkable because it would be well removed from the expedition route.